A preferred form of projection lenses for wide screen television is disclosed in U.S. Pat. Nos. 4,348,018 and 4,300,817 as well as co-pending U.S. application Ser. No. 280,785 now U.S. Pat. No. 4,526,442, all assigned to the assignee of the present application.
The lenses disclosed in the aforementioned patents and application generally comprise three groups: from the image end a first group of relatively small optical power serving to primarily correct aperture dependent aberrations; a second group which supplies substantially all the positive power of the lens; and a third group having a strongly concave surface towards the image end of the lens, serving as a field flattener, and essentially correcting any Petzval curvature of the first and second groups. As used in the foregoing mentioned patents, the term "lens group" refers to a combination of one or more lens elements which provide a specified optical function(s). In accordance with present U.S. Patent and Trademark Office practice, such lens groups are hereinafter referred to as "lens units" with the same meaning. The lenses, as disclosed, are designed for use with a surface of a cathode ray tube. The lenses of U.S. Pat. No. 4,300,817, disclosing a single biconvex element in the second lens unit, all have an equivalent focal length (EFL) of about one hundred fourteen millimeters or greater, while the lenses of U.S. Pat. No. 4,348,081, which utilize a two-element second lens unit including the biconvex element, may have reduced EFL's. The lenses described in U.S. Pat. No. 4,526,442 are designed to have a fold in the optical axis between the first and second lens units and have been designed so that the EFL is as low as one hundred fourteen millimeters. These EFL's are for cathode ray tube screens having a display surface with an approximate five inch diagonal.
Projection TV sets are rather bulky and require high volume cabinets. One manner of reducing the cabinet size is to to decrease the EFL of the projection lenses.
A further consideration is introduced wherein a spacing is provided between the screen of the CRT and the third lens unit of the projection lens. This spacing may be required for the inclusion of a liquid cooling material and a window necessary to enclose the coolant against the face of the CRT. This additional spacing between the face of the CRT causes the third negative element to contribute more negative power, which must be compensated by increased power in the positive second lens unit.
An effect of increasing the angular coverage of the lens as a result of decreasing the EFL, is that the aberrations become more difficult to correct. A single biconvex element second lens unit, as shown in the aforementioned patents, does not provide the lens designer adequate degrees of freedom to correct for the resulting astigmatism and distortion. By dividing the optical power of the second lens unit, as disclosed in U.S. Pat. No. 4,348,081, the EFL may be shortened. However, merely splitting the optical power of the second lens unit into two elements to obtain additional degrees of design freedom, does not provide acceptable contrast and resolution where the angular coverage of the projection lenses is required to be in excess of twenty-seven degrees semi-field.
The requirement that there be a fold in the optical axis between the first and second lens units to a mirror, as shown in the aforementioned co-pending application, requires that a large space be designed between the first and second lens units. This requirement further complicates the correction of astigmatism. In effect, the large air space between the first and second lens units eliminates a degree of design freedom, thus reducing contrast and resolution. The EFL of the lens is, of course, a function in the total conjugate distance between the CRT and the display screen. This is shown by the relationship EQU OL=EFL(1+1/M)+EFL(1+M)
where OL is the overall conjugate distance of the system from object to image
EFL (1+1/M) is the object distance PA1 EFL (1+M) is the image distance PA1 M is the magification of the system expressed as the ratio of object height to image height.
and
Therefore, in order to decrease the total distance between the cathode ray tube and the screen, it is necessary to reduce the EFL.
The present invention overcomes the aforementioned problems by forming the second lens unit in a particular configuration which allows shortening the EFL of the Lens while providing the necessary correction.